Microprocessor Systems: Addition, Subtraction and Flags in EE222, Essays (university) of Microprocessor and Assembly Language Programming

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Addition, Subtraction

EE222 – Microprocessor Systems Arbab Latif Spring 2021 Resources: The Intel Microprocessors: Architecture, Programming, and Interfacing, Eighth Edition Barry B. Brey ( Section 5.1 )

5 - 1 ADDITION, SUBTRACTION AND COMPARISON

• The bulk of the arithmetic instructions found in any microprocessor include addition, subtraction, and comparison.
• Addition, subtraction, and comparison instructions are illustrated.
• Also shown are their uses in manipulating register and memory data.

List of Each Flag bit, with a brief

description of function.

• C (carry) holds the carry after addition or borrow after subtraction. - also indicates error conditions
• P (parity) is the count of ones in a number expressed as even or odd. Logic 0 for odd parity; logic 1 for even parity. - if a number contains three binary one bits, it has odd parity; If a number contains no one bits, it has even parity

• A (auxiliary carry) holds the carry (half-carry) after addition or the borrow after subtraction between bit positions 3 and 4 of the result.
• Z (zero) shows that the result of an arithmetic or logic operation is zero.
• S (sign) flag holds the arithmetic sign of the result after an arithmetic or logic instruction executes.
• T (trap) The trap flag enables trapping through an on-chip debugging feature.

• IOPL used in protected mode operation to select the privilege level for I/O devices.
• NT (nested task) flag indicates the current task is nested within another task in protected mode operation.
• RF (resume) used with debugging to control resumption of execution after the next instruction.
• VM (virtual mode) flag bit selects virtual mode operation in a protected mode system.

• AC, (alignment check) flag bit activates if a word or doubleword is addressed on a non- word or non-doubleword boundary.
• VIF is a copy of the interrupt flag bit available to the Pentium 4– (virtual interrupt)
• VIP (virtual) provides information about a virtual mode interrupt for (interrupt pending) Pentium. - used in multitasking environments to provide virtual interrupt flags

Addition

• Addition (ADD) appears in many forms in the microprocessor.
• A second form of addition, called add-with- carry , is introduced with the ADC instruction.
• The only types of addition not allowed are memory-to-memory and segment register. - segment registers can only be moved, pushed, or popped
• Increment instruction (INC) is a special type of addition that adds 1 to a number.

Register Addition

• When arithmetic and logic instructions execute, contents of the flag register change. - interrupt, trap, and other flags do not change
• Any ADD instruction modifies the contents of the sign, zero, carry, auxiliary carry, parity, and overflow flags. - Only the flags located in the rightmost 8 bits of the flag register and the overflow flag 0000 03 C3 ADD AX,BX 0002 03 C1 ADD AX,CX 0004 03 C2 ADD AX,DX

Memory-to-Register Addition

• Moves memory data to be added to the AL (and other) register.

Array Addition

• Memory arrays are sequential lists of data.
• 10 Byte data [0-9]
• Add content of array element 3,5 and 7

Increment Addition

• The INC instruction adds 1 to any register or memory location, except a segment register.
• The size of the data must be described by using the BYTE PTR, WORD PTR, DWORD PTR, or QWORD PTR directives.
• The assembler program cannot determine if the INC [DI] instruction is a byte-, word-, or doubleword-sized increment.

Addition-with-Carry

• ADC) adds the bit in the carry flag (C) to the operand data. - mainly appears in software that adds numbers wider than 16 or 32 bits in the 80386–Core - like ADD, ADC affects the flags after the addition